1. Technical Field
The present invention relates to a plating method and apparatus for sequentially performing a plating process on a substrate such as a lead frame, a printed board, a flexible circuit, a tape substrate, or a semiconductor wafer used for electronic devices, and more particularly to a plating method and apparatus used to deposit a metal film on a substrate for large-scale integrated circuits (LSI) or to form interconnections in such a substrate. Further, the present invention relates to a substrate processing method for sequentially performing a wet process, such as a plating process, a coating process, or an etching process, on fine recesses or trenches formed in a substrate.
2. Background Art
Recently, a plating process has been employed to form metal films, organic films, interconnections, or bumps (protruding connecting electrode terminals) for semiconductor circuits in a substrate such as a silicon wafer. For example, it has widely been practiced to form bumps of gold, silver, copper, solder, nickel, or multi-layer materials of these metals at predetermined portions on a surface of a semiconductor wafer, which has semiconductor circuits and fine interconnections between the semiconductor circuits, to electrically connect the interconnections via the bumps with electrodes of a package substrate or with tape automated bonding (TAB) electrodes.
Methods of forming interconnections or bumps include various methods, such as electroplating, electroless plating, vapor deposition, and printing. According to a recent tendency to an increased number of I/O terminals in a semiconductor chip and to finer pitches between interconnections, an electroplating method has been employed more frequently because of its capability of fine processing and a high deposition rate. The electroplating method, which is one of the most popular methods of forming interconnections or bumps, can form a metal film having a high purity at a high deposition rate by a relatively simple control. Meanwhile, an electroless plating method does not need a seed layer on a substrate to supply an electric current. Thus, the electroless plating method requires a smaller number of processes to form interconnections or bumps in the substrate.
When a film is to be formed on a semiconductor substrate, it is strongly desired to improve uniformity of the thickness and the quality of the film and reduce manufacturing cost. Accordingly, various improvements have been developed so far in both of the plating methods.
In order to achieve satisfactory plating without defects, a cleaning process is performed on a surface (to be plated) of a substrate before a plating process in many cases. Such a process to remove dirt or oxides on a surface of a substrate before a plating process is referred to as a pre-plating process. Substrates for LSI or fine circuit boards are generally handled in a clean environment. Nevertheless, their circuit patterns are so fine that serious defects may be caused by slight change in quality of a surface of a substrate or slight attachment of foreign matter to a surface of a substrate. Accordingly, in many cases, a pre-plating process is performed immediately before a plating process.
Further, as one of the methods of forming circuits in substrates for LSI or fine circuit boards, there has widely been employed a method of forming a photopolymer film (resist or photoresist) on a substrate, transferring a circuit pattern or a bump pattern to the resist by a pattern transferring device such as a pattern exposure device, and then plating the substrate to form interconnections or bumps in the substrate. In this case, after a pattern has been transferred to a surface of a resist, a development process is performed to remove the resist at desired portions. Thus, a plating pattern is formed so that an underlying film is exposed at portions at which the resist has been removed. The underlying film has surfaces brought into contact with a plating solution. These surfaces of the underlying film should be clean such that no foreign matter is present thereon.
Generally, photopolymer resists are unlikely to be wet with a plating solution during plating. Specifically, resists have poor wettability. On the contrary, some resists have high water repellency. If a resist has poor wettability, air bubbles are often produced and left in fine patterns in the resist so as to cause plating defects. Specifically, if a resist has poor wettability, plating defects are likely to be caused. Particularly, according to a recent tendency to an increased number of I/O terminals in a semiconductor chip and to finer pitches between interconnections, a pre-plating process has increasingly become important to remove air bubbles in fine patterns of a substrate.
When a plated film is to be formed in fine trenches or plugs for interconnections in a substrate such as a semiconductor wafer or in openings of a resist having poor wettability, a plating solution or a pre-treatment liquid cannot sufficiently enter the trenches, plugs, or openings. Thus, air bubbles are likely to be produced in the plating solution or the pre-treatment liquid and left in the fine trenches, plugs, or openings. Such air bubbles may cause plating defects or plating incompletion.
In order to prevent such plating defects or plating incompletion, it has been proposed to add a surface-active agent to a plating solution so as to lower surface tension of the plating solution to thereby facilitate entry of the plating solution into fine trenches or plugs for interconnections in a substrate or openings of a resist. However, when the surface tension of the plating solution is lowered, air bubbles are likely to be produced in a plating solution during circulation. Further, addition of the surface-active agent to the plating solution may cause abnormal plating deposition or increase the amount of organic matter contained in a plated film. Thus, properties of the plated film may adversely be affected by addition of the surface-active agent to the plating solution.
Accordingly, as described above, it is necessary to remove air bubbles from a surface of a substrate before a plating process so as to achieve satisfactory plating without defects. Substrates for LSI and fine circuit boards are generally handled in a clean environment. Nevertheless, their circuit patterns become so fine that serious defects may be caused by slight attachment of air bubbles to the surface because photopolymer resists are generally hydrophobic. Accordingly, a pre-plating process such as a deaeration process should be performed immediately before a plating process to wet fine trenches or plugs for interconnections in a substrate or openings of a resist sufficiently with a plating solution.
As described above, the surfaces of the underlying film should be clean such that no foreign matter is present thereon. Specifically, since plating defects may be caused by attachment of a resist residue or foreign matter to the surfaces of the underlying film exposed to the outside, it is necessary to perform a process for removing the resist residue or foreign matter, e.g. a descum process.
Generally, for such a descum process, a plasma ashing method has been employed. The plasma ashing method includes reacting a resist residue or organic foreign matter with activated oxygen to convert it into carbon dioxide (CO2), water vapor (H2O), and the like to remove the resist residue or organic foreign matter. The plasma ashing process is a dry process which is performed in a clean vacuum. Accordingly, a plasma ashing device for performing the plasma ashing process is generally located at a different site than a device for performing a chemical liquid process (wet process) such as a plating process.
In general, a plating apparatus includes a plating tank (plating chamber) and a pre-treatment tank (pre-treatment chamber) for performing a pre-plating process using an acid or alkali solution. In the pre-treatment tank of the plating apparatus, a cleaning process or an activation process is performed on a surface of a substrate (surfaces of an underlying film). A substrate for LSI or a fine circuit board is immersed in a pre-plating liquid immediately before a plating process to clean and activate the surface of the substrate. The substrate is cleaned (or rinsed) with pure water to remove the chemical liquid thereon. Then, the substrate is brought into contact with the plating solution to perform a plating process. Thus, the plating apparatus generally includes a pre-plating tank (pre-treatment tank), a rinsing tank, and a plating tank, which are arranged adjacent to each other.
In a plating method for forming interconnections or bumps in a substrate as described above, pre-plating processes are performed. Recently, in order to achieve further integration of semiconductor circuits or high-density packaging, it is desired to improve a conventional film deposition method using plating and provide a plating method without any defects. It is also desired to provide a plating apparatus having high reliability which can achieve such a plating method.
Further, if organic matter or a resist residue is attached to surfaces of an underlying film of the substrate which are exposed at bottoms of the resist pattern, then plating defects are more likely to be caused. Accordingly, there are strongly desired a plating method which can solve these drawbacks and an apparatus which can achieve such a plating method.